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Tiny hopper With voices hardly louder than an insect's buzz, the tiniest frogs ever discovered are smaller than a coin and hop about the rainforest of Papua New Guinea, report US scientists.

Not only are these little peepers with the big names - Paedophryne amauensis and Paedophryne swiftorum - the smallest frogs known, they are also believed to be the smallest vertebrates on Earth, according to a report published in the science journal PLoS ONE .

Until now the smallest vertebrate was believed to be a transparent Indonesian fish known as Paedocypris progenetica that averaged about eight millimetres.

The largest vertebrate is the blue whale, measuring about 25.8 metres.

The little land frog Paedophryne amauensis comes in at a whopping 7.7 millimetres. The other newly discovered kind, Paedophryne swiftorum, measures a bit over eight millimetres.

"It was particularly difficult to locate Paedophryne amauensis due to its diminutive size and the males' high pitched insect-like mating call," says Louisiana State University scientist Chris Austin, who discovered them.

"But it's a great find."

The dark brown frogs with bluish-white specks have likely existed for a long time, underfoot and out of sight on the rainforest floor, eating smaller prey or being eaten by bigger predators.

"The ecosystems these extremely small frogs occupy are very similar, primarily inhabiting leaf litter on the floor of tropical rainforest environments," says Austin.

"We now believe that these creatures aren't just biological oddities, but instead represent a previously undocumented ecological guild - they occupy a habitat niche that no other vertebrate does."

In fact, judging by the frequency of male mating calls they heard, Austin says the tiny frogs might be spaced as close as 50 centimetres from each other on ground beneath the leaves.